In case of executing a coating process on a surface of various substrate sheets, dispersion slurry with solid particulates dispersed in dispersion medium, a partial change in concentration of each components occurs due to phase separation, precipitation, and aggregation, unlike in the case that a liquid to be coated is a uniform solution. When a dispersion slurry is coated, of various coating apparatuses, it is preferred that a roll-coating apparatus, which is relatively simple in structure and low in equipment cost, is used. However, a conventional method of roll-coating is prone to cause precipitation of solid particulates in the dispersion slurry or their adhesion to the surface of the coating roll. As a result, uneven coating surface and clogging during a job run among other things, occur, making a uniform and stable coating process difficult.
In order to alleviate adhesion of dispersion slurry to the roll surface as described above, ingenious approaches, including providing a surface finish on the surface of the coating roll with strippable material such as silicone and Teflon™, scraping off by incorporating a scraper, creating a strippable condition by rotating the roll in a forward or a backward direction, etc., have been performed.
The conventional countermeasures, however, have not fully solved the above problems that the dispersion slurry adheres to the coating roll surface. Such the adhesion is a common phenomenon in contact-type coaters, including not only roll-coaters but also bar-coaters and knife-edge-coaters where a coater directly contacts with a substrate. In order to fundamentally prevent such troubles, in the conventional level of technology, there is no alternative but to employ a non-contact-type coater, such as a curtain-coater, grid-coater, or spray-coater, in place of a contact-type coater. This, however, will newly add limiting factors to the properties of dispersion slurry for coating and complicate the apparatus for coating, and greatly increase the equipment cost as well. Especially, in the case of the dispersion slurry, with water-absorbing solid particulates called “SAP” dispersed in the water-organic-solvent-mixture dispersion medium, such as the subject of the present invention, because of the strong surface tack of the water-absorbing solid particulates, significant troubles tend to occur in the conventional non-contacting type of coating apparatus, making it extremely difficult to execute a uniform and stable coating process.
On the other hand, the obtained super-absorbent sheet will not be able to offer satisfactory properties in the case that the sheet is applied for diapers, feminine protection products and other sanitary products, even if a super-absorbent coating layer is formed on the substrate sheet by overcoming the aforementioned difficulties. One of the reasons is that if a super-absorbent coating layer is provided so as to entirely cover one of the surfaces of the substrate sheet, even if the substrate sheet is as soft and highly flexible as non-woven fabric, the flexibility required of the water-absorbing sheet is lessened due to stiffness of the super-absorbent coating layer and thus it cannot meet the performance criteria sought for the above sanitary products. Furthermore, a super-absorbent sheet comprising a super-absorbent coating layer entirely covering one of the surfaces of the substrate sheet may have an adequate water absorbing capacity, and an absorbing speed of the sheet depends on that of the coating layer. As a result, when the liquid exceeding the absorbing speed in volume is supplied, the liquid which has failed to absorb in the absorbing layer/the sheet will remain there, thus becoming the cause of leakage.
The inventors of the present invention have proposed that in order to correct these shortcomings, a technique of coating the surface of the substrate sheet while leaving a part of it uncoated (as in for example, Japan Patent Laid-open 10-168230A and Japan Patent Laid-open 2000-005674A). In this technique heretofore known, a water-absorbing layer in multiple band shape extending along the longitudinal direction of the substrate sheet is formed by supplying the slurry in multiple band shape on the surface of the moving substrate sheet. The region where water-absorbing coating layer is present and the region where it is not present, or in other words the region where the substrate sheet is exposed, are mixed on the surface of this super-absorbent sheet. As the result, the liquid which has failed to be absorbed in the water-absorbing coating layer will be diffused in the region where the water-absorbing coating layer is not present, to other areas by the substrate sheet's diffusivity, thereby shortening the time of the liquid's retention and, as a result, alleviating the problem of leakage.
The water-absorbing coating layer is continuous in the longitudinal direction, though it is discontinuous in the width direction of the substrate sheet. Therefore, diffusivity and flexibility are obtained in the width direction, but not in the longitudinal direction, making it impossible to obtain satisfactory performance.
Such defects in the longitudinal direction can be solved by intermittently supplying the dispersion slurry, when coating, to make the coating layer in the longitudinal direction discontinuous. Furthermore, a discontinuous coating layer both in width and longitudinal directions can be achieved by combining it with a technique to make it discontinuous in the width direction, as described above.